Radio communication method between a base station and mobile terminals

ABSTRACT

Radio signals are transmitted between a base station and mobile terminals. The radio signals are transmitted from the base station to the mobile terminals on a first frequency and are transmitted from the mobile terminals to the base station on a second frequency. The first frequency carries a downlink traffic channel, while the second frequency carries an uplink traffic channel. The first frequency further carries a first downlink signaling channel to a mobile terminal in the transmission phase on the uplink traffic channel, and a second downlink signaling channel to a mobile terminal in the reception phase on the downlink traffic channel. The second frequency further carries a first uplink signaling channel from a mobile terminal in the reception phase on the downlink logical traffic channel and a second uplink signaling channel from a mobile terminal in the transmission phase on the uplink traffic channel.

TECHNICAL FIELD

The present invention relates to the field of digitalradiocommunications between base stations and mobile terminals, usingfrequency duplexing of logical traffic channels.

It is aimed especially at applications in professionalradiocommunications.

BACKGROUND OF THE INVENTION

In a radiotelephony system, the mobile terminals must be capable ofreceiving signaling while a call is set up on a traffic channel, so asto exchange information relating to the services offered to theterminals or to the management of the radio links.

In frequency-duplexing cellular systems such as GSM, signaling channelscalled SACCH are associated with the traffic channels. They use the samecarrier frequencies as the traffic channels, via a time-divisionmultiplexing mechanism. These signaling channels, like their associatedtraffic channels, are in full duplex mode.

This is not well suited to the needs of professional radiocommunicationsystems, which frequently operate in alternate mode, and for which thegroup communications play a very important role. In such a system, aknown solution consists in making provision for a pair of controlfrequencies which are common to all the terminals attended to by a basestation, and over to which the communicating mobile terminalsperiodically transfer for the needs of the associated signalingchannels. This solution is not very flexible. Furthermore, it demandssynchronism between the traffic frames and the control frames, thishaving the drawback of prohibiting the monitoring and presynchronizationmechanisms which allow better management of radio resources, inparticular through the effective implementation of intercell handovers.

Patent Application EP-A-0 896 443 describes a system ofradiocommunication with mobile terminals exhibiting the feature ofoffering time-division multiplexing services with various degrees ofprotection related to the possible use of modulation coded on thecarrier. For a given throughput, offered for the execution of theservice, the number of timeslots allotted to the service is related tothe coding or to the absence of coding of the modulation, and/or to therate of the coding applied. In a particular execution of this system,one and the same service can be offered in a first mode on ahalf-channel with noncoded modulation, or in a second mode on a fullchannel with modulation coded by a code of rate ½.

Patent application EP-A-0 677 930 describes a frequency-divisionmultiplexing radiocommunication system in which a carrier frequencysupports a radio signal organized as multiframes comprising trafficframes and signaling frames, uplink or downlink, reserved for the mobilestations in the transmission mode.

Patent Application EP-A-0 644 702 describes a time-division multiplexingradiocommunication system in which time intervals of frames shiftedbetween the uplink and the downlink support logical signaling channels,uplink or downlink, for mobile stations in the transmission mode orreception mode.

The aim of the present invention is to propose an organization ofassociated signaling channels which properly meets the needs ofprofessional radiocommunication systems.

SUMMARY OF THE INVENTION

The invention thus proposes a method of transmitting radio signalsbetween a base station and mobile terminals, on a first frequency forsignals transmitted from the base station to at least one of the mobileterminals and on a second frequency for signals transmitted from atleast one of the mobile terminals to the base station, the transmissionson the first and second frequencies being performed according to a framestructure comprising timeslots of like duration. The first frequencycarries a downlink logical traffic channel to at least one of the mobileterminals, while the second frequency carries an uplink logical trafficchannel from one of the mobile terminals. The first frequencyfurthermore carries a first downlink logical signaling channel to one ofthe mobile terminals in the transmission phase on the uplink logicaltraffic channel, and a second downlink logical signaling channel to atleast one of the mobile terminals in the reception phase on the downlinklogical traffic channel. The second frequency furthermore carries afirst uplink logical signaling channel from at least one of the mobileterminals in the reception phase on the downlink logical traffic channeland a second uplink logical signaling channel from one of the mobileterminals in the transmission phase on the uplink logical trafficchannel. The first downlink and uplink logical signaling channels areeach carried by a first timeslot of the frame structure, while thesecond downlink and uplink logical signaling channels each occupy asecond timeslot of the frame structure, distinct from said first slot.

In a particular embodiment of the method, the frame structure comprisestimeslots dedicated to the logical traffic channels, at least onetimeslot dedicated to the logical signaling channels and at least onetimeslot forming for the mobile terminals a window for listening tosignals transmitted from other base stations.

Preferably, the frame structure comprises a programmable timeslotscheduled for the transmission on at least the first uplink logicalsignaling channel, the base station transmitting, in the course of thetimeslots dedicated to the logical traffic channels, signals forcontrolling access to said programmable timeslot by the mobile terminalsin the reception phase. It is thus possible to adjust according to needsthe throughput of the downlink and uplink control channels between thebase station and the terminals in the reception phase on the trafficchannel. Said access control signals may comprise signals forauthorizing the mobile terminals in the reception phase to transmit inthe course of a next programmable timeslot, and signals for apprising ofthe processing of requests made by the mobile terminals in the receptionphase in the course of a previous programmable timeslot.

When the logical traffic channels are used in alternate mode, withsuccessive alternations in the course of which only one of the mobileterminals is in the transmission phase on the uplink logical trafficchannel while one or more of the mobile terminals are in the receptionphase on the downlink logical traffic channel, it is possible for thebase station to transmit access control signals in such a way as torestrict the transmissions of the mobile terminals in the course of theprogrammable timeslots in an initial period of an alternation withrespect to the rest of the alternation.

Another aspect of the present invention relates to a radiocommunicationbase station, comprising means for transmitting and for receiving radiosignals having the above structure as regards the associated logicaltraffic and signaling channels.

A third aspect of the present invention relates to a radiocommunicationmobile terminal, comprising means for transmitting and receiving radiosignals, having the above structure as regards the associated logicaltraffic and signaling channels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary base station according tothe invention;

FIG. 2 is a schematic diagram of an exemplary mobile terminal accordingto the invention;

FIG. 3 is a chart illustrating the frame structure transmitted onphysical control channels formed in one exemplary embodiment of theinvention;

FIGS. 4 and 5 are charts respectively illustrating two frame structurestransmitted on traffic channels formed in an exemplary embodiment of theinvention;

FIGS. 6 and 7 are charts detailing the respective structures of twotimeslots of the frame of FIG. 3;

FIG. 8 is a chart illustrating an alternative frame structuretransmitted on physical control channels;

FIGS. 9 and 10 are charts detailing the respective structures of twotimeslots of the frame of FIG. 8; and

FIG. 11 is a chart illustrating another alternative frame structuretransmitted on physical control channels.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the embodiment described here by way of example, the base station andthe mobile terminal represented in FIGS. 1 and 2 belong to aprofessional radiocommunication system operating in frequency divisionmultiple access mode (FDMA). It is assumed, by way of illustration, thatthis system implements the method of defining channels described in theaforesaid patent application EP-A-0 896 443, using for one and the sameservice either a full channel with modulation coded by a code of rate1/K (mode 2), or a fractional channel of K times smaller throughput witha no-coded modulation (mode 1), with K=2. One then takes intoconsideration elementary timeslots, whose duration d₁ is for example 20ms, used in mode 1, and composite timeslots, whose duration d₂=K.d₁ isin this example 40 ms, used in mode 2.

For each base station there is defined, on a particular frequencyf_(CD), a downlink physical channel devoted to the transmission ofcontrol information. Symmetrically, an uplink physical channel isdefined on a frequency f_(CU) for transmitting control information fromthe mobile terminals to the base station. These physical controlchannels are subdivided into logical control channels by time-divisionmultiplexing. Some of these logical channels are common channels sharedby the mobile terminals within range of the base station. Others arededicated channels, which the base station uses to communicate withparticular mobiles.

The signal transmitted on each of the physical control channels takesthe form of successive frames subdivided into K.M elementary timeslotsbelonging to different logical channels. In the example illustrated byFIG. 3, where M=13, the elementary slots denoted F, S0 and P relate tocommon downlink channels, and those denoted Si (with 1≦i≦11) relate tobidirectional dedicated channels.

The slots F have a duration d₁′ and are repeated every K′.M elementarytimeslots, with d₁′=d₁ and K′=K=2 in the example of FIG. 3. They containa synchronization pattern formed by a predetermined sequence of bitsmaking it possible to carry out the frequency- and time-synchronizationof the mobile terminals.

The slots S0 have a duration d₁′ and are repeated every K′.M elementarytimeslots. They contain system information required for coordinationbetween the mobiles and the base station, comprising for example: (i) afield H of 5 bits tagging the position of the timeslot S0 in the currentsuperframe (a superframe represents the smallest common multiple betweenthe periodicity of the traffic channels and that of the controlchannels, i.e. 13×27 composite timeslots in the example considered, i.e.14.04 s); (ii) a field X of 3 bits tagging the position of the timeslotS0 in a longer period (hyperframe), such as a period of encryption ofthe air interface (typically of the order of an hour); and (iii) a fieldR of 3 bits indicating the minimum field strength received for access tothe cell (for example quantized in steps of 5 dB).

The slots P are used by the base station for addressing messages tomobile terminals with which it is not currently communicating (paging).In the uplink direction, the elementary timeslots left blank in FIG. 3,or those denoted Si (1≦i≦11) which are not allotted as dedicatedchannels, can be used by the mobile terminals to perform random access(common uplink channel).

The slots Si (1≦i≦11) of the dedicated channels are used after anallocation procedure. They each arise twice per frame in the exampleconsidered. The control frame being 520 ms long, a timeslot Si, forgiven i, occurs on average every 260 ms, with a duration of 100 msbetween the transmitting of a message by the base station over adownlink slot Si and the transmitting of the response by the mobileterminal over the next uplink slot Si, and a duration of 140 ms or 180ms between the transmitting of a message by the mobile terminal over anuplink slot Si and the transmitting of the response by the base stationover the next downlink slot Si.

The base station can furthermore set up traffic channels with one ormore mobile terminals situated within its range, after a setup procedureperformed by means of a dedicated control channel Si. The trafficchannel set up with a terminal is downlink (frequency f_(TD)) and/oruplink (frequency f_(TU)) The traffic channel is multiplexed on thefrequency f_(TD) and/or f_(TU) with associated signaling channelsserving to exchange signaling during communication (for examplemeasurements or commands for controlling the radio power transmitted bythe mobiles, call signaling, requests and commands for changing cell,pre-empting alternate operation, etc.).

The uplink and downlink traffic channels may have the frame structurerepresented in FIG. 4 corresponding to mode 1, or that represented inFIG. 5 corresponding to mode 2. Each frame of the traffic channel has aduration corresponding to K.Q=54 elementary timeslots (Q=27), and isdivided into three parts of 18 elementary slots. In each of these threeparts, the first eight composite timeslots are occupied by the logicaltraffic channel. The ninth composite timeslot is occupied by associatedcontrol channels for the first two parts, and unoccupied for the thirdpart. This unoccupied slot, hatched in FIGS. 4 and 5, constitutes amonitoring window during which the mobile terminal changes frequency soas to observe the physical control channels of the base stations of theneighboring cells, so as to be able to perform a change of cell ifnecessary.

In mode 1 illustrated by FIG. 4, each of the first eight compositetimeslots of each third of the frame comprises an odd elementary slotfor the downlink direction and an even elementary slot for the uplinkdirection, tagged by the letter T in the figure. Consequently, on thesame downlink carrier f_(TD), the base station can multiplex a logicaltraffic channel set up with another mobile terminal. Furthermore, if themobile terminal is capable of passing from frequency f_(TD) to frequencyf_(TU) and vice versa in the short timespan separating two elementaryslots, mode 1 makes it possible to set up the communication intime-duplex mode.

In mode 2 illustrated by FIG. 5, the composite timeslots of the framestransmitted on the traffic channels are not subdivided into twoelementary slots. The signal, transmitted with the same informationthroughput, is subjected to a modulation coded with a rate of 1/K=½ asset forth in patent application EP-A-0 896 443, thereby achieving bettersensitivity for the receiver. With this mode of operation, the timeduplex described earlier cannot be used. In the general case where themobile terminals are not capable of modulating and of demodulatingsimultaneously about two different carrier frequencies, this mode ofoperation necessitates a communication discipline of alternate operationtype.

Within the monitoring window of a traffic frame, the mobile terminalattempts to detect the synchronization pattern transmitted in thetimeslot F of the control frame by the base station of a neighboringcell. It therefore demodulates the signal received according to thefrequency f_(CD) used in this neighboring cell. If the synchronizationpattern is detected, the terminal uses the same frequency f_(CD) in themonitoring window of a following frame, and attempts to extract thesystem information transmitted by the same base station in its slot S0.If this information is properly received, the mobile terminal is readyto change cell if necessary.

The monitoring window has a duration d₂ corresponding to a compositetimeslot, i.e. K=2 elementary slots. In order to be certain that in thecourse of a superframe, these windows cover the timeslots F and S0 ofthe downlink control frames of the neighboring cells, it is judiciousfor the periodicity of these time windows, and that of the timeslots Fand S0 on the physical control channel, expressed as a number ofcomposite timeslots, to be mutually prime. Stated otherwise, theperiodicity of the monitoring windows being Q composite slots, and thatof the timeslots F and S0 being M composite slots, the numbers M and Qare chosen to be mutually prime, this being the case in the embodimentdescribed where M=13 and Q=27. The mobile terminal then monitors thevarious possible frequencies f_(CD) at the superframe rate, until thesynchronization pattern transmitted in a neighboring cell is detected.

Furthermore, the timeslot S0 occurring p composite slots after thetimeslot F on the carrier f_(CD), with p<M (p=1 in the example of FIG.3), it is judicious to choose the integer Q of the form q.M+p, with qinteger. This condition is fulfilled in the example described where p=1,q=2, M=13 and Q=27. Hence, when the mobile terminal picks up thesynchronization pattern transmitted by a cell in a monitoring window, itcan pick up the system information transmitted by this same cell rightfrom the next monitoring window, thereby minimizing the duration of theacquisition process.

In FIG. 1, block 30 denotes the source of the synchronization patterntransmitted in the slots F, and block 31 the source of the systeminformation transmitted in the slots S0. Block 32 diagrammaticallydepicts the circuits serving to process the information exchanged on theother common control channels, in particular paging and random accesschannels. The block 33 diagrammatically depicts the circuits devoted tothe processing and exchanging of information on the dedicated controlchannels S1-S11 set up with various mobile terminals in the cell. Amultiplexer 35 receives the signals delivered by blocks 30 to 33 andconstructs the downlink frames represented in the upper part of FIG. 3under the control of a module 36 for synchronizing and managing theframes. The output stream from the multiplexer 35 is provided to amodulator 37 which carries out the modulation about the carrierfrequency f_(CD) output by the frequency synthesis module 38.

For reception on the control channel, the base station comprises ademodulator 49 which demodulates the signal received in relation to thecarrier frequency f_(CU) provided by the module 38, and delivers to thedemultiplexer 51 the downlink binary frames having the structurerepresented in the lower part of FIG. 3. Under the control of the module36 for synchronizing and managing the frames, the demultiplexer 51extracts the information relevant to the common control channels 32 andthe dedicated control channels 33.

In addition to the physical control channel, the base station can set upa certain number of traffic channels with mobile terminals situatedwithin its range. In the simplified example represented in FIG. 1, thebase station is regarded as using a single traffic frequency f_(TD) inthe downlink direction and a single traffic frequency f_(TU) in theuplink direction, the block 40 designating the circuits, supervised bythe module 36, serving for the processing and exchanges on these trafficchannels and on the associated control channels.

A modulator 41 modulates the digital signal produced by the block 40,which has the structure represented in the upper part of FIG. 4 or 5,about the carrier frequency f_(TD) delivered by the frequency synthesismodule 38. The frequency f_(TU) of the uplink traffic channel isreceived from the synthesis module 38 by a demodulator 50. The resultingdigital signal, which has the structure represented in the lower part ofFIG. 4 or 5, is addressed to the processing circuits 40 of the trafficchannel.

When a traffic channel has been allotted, the frame synchronization andmanagement module 36 instructs the modulator 41 and the demodulator 50to activate the coding of the modulation and the deployment of thecorresponding demodulation scheme only if mode 2 is required (FIG. 5).

In practice, to ensure multiple access, the base station will compriseseveral modulators 41 and several demodulators 50 operating according tothe various traffic frequencies.

The radio signals delivered by the modulators 37 and 41 are combined bythe summator 42. The resulting signal is converted into analog at 43,then amplified at 44 before being transmitted by the antenna 45 of thebase station. A duplexer 46 extracts the radio signal picked up by theantenna 45 of the base station, and supplies it to an amplifier 47.After digitization 48, the signal, received and amplified, is suppliedto the demodulators 49 and 50.

A mobile terminal communicating with the above base station can complywith the schematic diagram of FIG. 2. The antenna 55 is linked to aduplexer 56 so as to separate the signals transmitted and received. Thesignal received is amplified at 57, then digitized at 58 before beingaddressed to the demodulator 59. The mobile terminal comprises a framesynchronization and management module 60, which controls the frequencysynthesis module 61 so that it provides the demodulator 59 either withthe frequency f_(CD) of a physical control channel, or the frequencyf_(TD) of a downlink traffic channel allotted to the terminal.

When the demodulator 59 operates at the frequency f_(CD), the digitalsignal frames, which may have the structure represented in the upperpart of FIG. 3 are addressed to a demultiplexer 64 controlled by thesynchronization module 60 so as to distribute the signals pertaining tothe various logical channels to blocks 65, 66, 67, 68 which designatethe circuits respectively used to detect the synchronization patterns onthe logical channel F, to extract the system information from thelogical channel S0, to process the common control channels and toprocess the dedicated control channel Si optionally allotted to theterminal. The frame synchronization and management module 60 alsocontrols a multiplexer 70 which forms the contribution of the terminalto the uplink frames at the frequency f_(CU) (lower part of FIG. 3).

When a traffic channel is allotted, the demodulator 59 operates at thefrequency f_(TD) (except in the monitoring windows), and its outputsignal is addressed to the circuits 71 which process the traffic channeland the associated control channels (reception of the channels DT, DL ofFIGS. 4 and 5). These circuits 71 furthermore deliver the stream to betransmitted on the frequency f_(TU), represented in FIG. 4 or 5 (trafficchannel and associated channels UL, UT).

The modulator 72 of the mobile terminal, controlled by the module 60,receives either the stream delivered by the multiplexer 70 and thefrequency f_(CU) for transmission on the physical control channel, orthe stream delivered by the circuits 71 and the frequency f_(TU) fortransmission on the traffic channel. The radio signal output by themodulator 72 is converted into analog at 73, into amplified at 74 beforebeing transmitted by the antenna 55.

When a traffic channel has been allotted, the frame synchronization andmanagement module 60 instructs the modulator 72 and the demodulator 59to activate the coding of the modulation and the deployment of thecorresponding demodulation scheme only if mode 2 is required (FIG. 5).

In the monitoring windows, the frame synchronization and managementmodule 60 of the terminal indicates to the frequency synthesis module 61the frequency f_(CD) to be supplied to the demodulator 59, distinct fromthe frequency f_(CD) of the serving base station. It furthermoreinstructs the demultiplexer 64 in such a way that the demodulated signalis addressed to block 65 for detecting the synchronization pattern. Ifthe synchronization pattern is not detected, the module 60 repeats thesame process in the course of the next monitoring window, until the samefrequency f_(CD) has been monitored M times. When the synchronizationpattern is detected in a monitoring window (data A in FIG. 2), themodule 60 causes the same frequency f_(CD) to be maintained in the nextwindow, and it instructs the demultiplexer 64 so that the demodulatedsignal is addressed to block 66 for extracting the system information.

A mobile terminal compatible with the mode of operation of FIG. 4, withthe time duplex, must have a frequency synthesis module 60 capable ofproviding two different frequencies f_(TD), f_(TU) at instants which arevery close to the boundaries between the elementary timeslots. Inpractice, this requires the module 60 to comprise two distinct frequencysynthesizers, this having a significant impact on the cost of theterminal.

A simplified version of the terminal does not allow alternation betweenthe frequencies f_(TD) and f_(TU) between the elementary timeslots. Thissimplified terminal operates under alternate mode only, since therelative slowness of frequency switching poses no problem at the momentof the alternations.

However, in the monitoring window of each frame, the synthesis module 60must be capable of switching its frequency so as to examine the physicalcontrol channels of the neighboring cells. When the timeslot F of aneighboring cell falls within the monitoring window of the mobileterminal tuned to the frequency of the control channel of thisneighboring cell, the terminal is at risk of failing to detect thesynchronization pattern by reason of its relative slowness of frequencyswitching and/or of the absence of synchronization between the cells.

To avoid this, the elementary timeslot F is subdivided into N subslotsof like duration in the course of which the synchronization pattern FFis transmitted repeatedly. These subslots are here called“presynchronization slots”. By way of example, it is possible to takeN=4, the duration of the presynchronization slots being d₁′/N=5 ms (seefigure 6). It will then be possible to detect the synchronizationpattern FF even if the mobile terminal lacks the demodulation of thestart of timeslot F.

The synchronization which can thus be achieved is incomplete in thesense that there is no regard as to which of the N occurrences of thesynchronization pattern has been detected.

Nevertheless, this incomplete synchronization makes it possible to lockthe frequency of the synthesis module 61 of the terminal and to positionthe mobile terminal on the structure of the control frames of theneighboring cell, with an offset of k.d₁′/N, where k is an unknowninteger lying between 0 and N−1. Returning to the same monitoringfrequency f_(CD) with a delay of Q composite timeslots with respect tothe instant of the start of the detected synchronization pattern, themonitoring window of the next frame will make is possible to pick uppart at least of the timeslot S0 containing the system information.

The base station also subdivides this elementary timeslot S0 into Nsubslots of like duration d₁′/N each containing, in addition to theaforesaid fields H, X and R, a field SS of n bits, where n is theinteger equal to or immediately greater than log₂N (n=2 in the exampleconsidered). These n bits contain the serial number of the subslot inthe timeslot S0, i.e. SS=0, 1, . . . , N−1 (FIG. 7). The fields H, X andR contain the same values in the various subslots of the same slot S0.These N subslots of the slot S0 are here called “synchronizationsupplement slots”.

A fixed offset of d₂ occurs between each presynchronization slot and asynchronization supplement slot associated therewith.

Block 66 of the mobile terminal extracts the system information (fieldsH, X, R) in one of their occurrences, temporally located with respect tothe occurrence of the previously detected synchronization pattern.Furthermore it obtains the serial number SS of this occurrence, andsupplies it to the frame synchronization and management module 60together with the content of the fields H, X and R. This serial numberSS allows the module 60 to determine the aforesaid integer k and toresolve the uncertainty which had existed in the time synchronizationperformed solely on the basis of detecting the synchronization patternFF. Having thus completed the synchronization, the mobile terminal isready to change cell should this be necessary.

Considering for example that the terminal needs 5 ms to change itsfrequency, the monitoring window of duration d₂=40 ms is truncated by 5ms at the start and at the end. Given that, in the example of FIG. 3,the sought-after synchronization pattern FF lies in an slot F of d₁′=20ms modulo a periodicity which is a multiple of 40 ms without synchronismwith respect to the frame rate to which the terminal is tuned, theduration of the presynchronization slot containing this pattern FF is atmost 5 ms if we wish to guarantee its detection. This justifies thechoice N=4 in the case considered.

If the cells were synchronized, only the switching time needing to betaken into account, the duration of the presynchronization slot could beas much as 10 ms (N=2). However, this type of synchronization is notusually employed since it poses network architecture problems.

By making provision for the pattern FF to be present in all the subslotsof the slot F as represented in FIG. 6, its probability of detection ismaximized. However, it should be noted that, in the example of FIG. 3,only the first and the last subslots of 5 ms of the slot F could playthe role of presynchronization slots containing the synchronizationpattern FF, the other two subslots possibly containing something else.This suffices to ensure that the pattern FF can be detected by themobile terminals. Under these conditions, only the first and the lastsubslots of the slot S0 could, similarly, play the role ofsynchronization supplement slots containing the system information. Onebit is then sufficient in the synchronization supplement slots toidentify what it involves and to make it possible to complete thesynchronization.

Alternatively, the duration d₁′ of the timeslots F and S0 is 40 ms,their rate of repetition being the same as before, namely M×d₂=520 ms.Stated otherwise, K′=d₂/d₁′=1. For example, in certain embodiments, thetimeslots F, P and Si (i≧0) are not apportioned into two on the physicalcontrol channels (carriers f_(CD) and f_(CU)), i.e. there is notime-division multiplexing of order 2 on these carriers. The controlchannels can also have a structure as depicted diagrammatically in FIG.8, this being similar to that represented in FIG. 3 with the followingdifferences:

-   -   the timeslots F and S0 are d₁′=d₂=40 ms, while the slots Si with        1≦i≦11 remain of d₁=20 ms;    -   the logical channel formed by the slots denoted S6 has a        throughput reduced by a half.

The timeslots F and S0 of 40 ms may then be subdivided into N=4 subslotsof 10 ms, thereby ensuring the detection of the pattern FF with afrequency switching time of 5 ms for the mobile terminals, withoutsynchronism between the cells.

This makes it possible to transmit more information in the logicalchannel S0. In particular, the field X can be lengthened so as to allowlonger periods of encryption (for example 24 to 48 hours), or theindication of an encryption key index or of an encryption algorithm tobe used.

It may be observed that only the subslots of 10 ms of ranks 1 and 3 (or2 and 4) of the slots F and S0 could in this case be presynchronizationand synchronization supplement slots carrying respectively the patternFF and the system information, the other two subslots possiblycontaining something else.

FIGS. 9 and 10 illustrate a possible structure of the slots F and S0 of40 ms, subdivided into N=4 subslots of 10 ms. In this example, where thesynchronization pattern FF is 9.5 ms, each subslot of the slot F or S0includes a 0.5 ms slot devoted to an access control (AC) logicalchannel. The first and the third subslots of the slot F arepresynchronization slots commencing with the 0.5 ms slot of the ACchannel, followed by the pattern FF, while the second and the fourthsubslots of the slot F are presynchronization slots commencing with thepattern FF followed by the 0.5 ms slot of the AC channel. Likewise, thefirst and the third synchronization supplement subslots commence withthe 0.5 ms slot of the channel AC, followed by the fields SS=0 or 2, H,X, R which occupy 9.5 ms, while the second and the fourthsynchronization supplement subslots commence with the fields SS=1 or 3,H, X, R followed by the 0.5 ms slot of the AC channel.

The other timeslots (P, Si with 1≦i≦11) of the physical control channelon the frequency f_(CD), which are of d₁=20 ms (FIG. 8), each comprisetwo 0.5 ms slots devoted to the AC channel, one situated at the startand the other at the end of the slot. They are therefore disposed in thesame way as in the two halves of the slots F and S0. In each of theseslots P, Si, the useful data, if any, occupy for example a central spanof 17.5 ms flanked by two synchronization words of 0.75 ms (which allowthe terminals to track synchronization with their serving cell) and bythe two 0.5 ms AC slots. The two 0.5 ms slots of the channel AC whichlie in a 20 ms slot of rank n of the downlink control channel carry fourbits X₁, X₂, Y₁ and Y₂, protected by a code of rate ½, the significanceof which is for example as follows:

-   -   X₁X₂=00: the 20 ms slot of rank n+j of the uplink physical        control channel f_(CU) is unavailable for random access by the        mobile terminals (because it is occupied by an allotted        dedicated channel Si);    -   X₁X₂=01: the 20 ms slot of rank n+j of the uplink channel f_(CU)        is available for random access in normal mode;    -   X₁X₂=10: the 20 ms slot of rank n+j of the uplink channel f_(CU)        is available for random access in protected mode (which are        processed differently from those in normal mode);    -   X₁X₂=11: reserved;    -   Y₁=Y₂=00: the base station indicates that it has detected and        correctly processed a random access performed by a mobile        terminal during the 20 ms slot of rank n−j′ of the uplink        channel f_(CU);    -   Y₁Y₂=01: the base station indicates that it has detected a        random access performed during the 20 ms slot of rank n−j′ of        the uplink channel f_(CU), without having been able to properly        conclude the processing of this random access, for a reason        likely to be distinct from a problem of collision between two        concurring random accesses;    -   Y₁Y₂=10: the base station indicates that it has detected a        random access performed during the 20 ms slot of rank n−j′ of        the uplink channel f_(CU), without having been able to properly        conclude the processing of this random access, likely on account        of a collision between two concurring random accesses (the        mobile terminals follow a separate repetition protocol depending        on whether or not there has been a collision);    -   Y₁Y₂=11: reserved.

The positive numbers j and j′ are for example equal to 3 or 4.

In the example illustrated by FIG. 11, the timeslot F, of d₂=40 ms, issubdivided into N=4 subslots F1, F2, F3, F4 and the timeslot S0 issubdivided into N=4 corresponding subslots S01, S02, S03, S04. As in thecase of FIGS. 9 and 10, the subslots of odd ranks F1, F3, S01, S03commence with 0.5 ms devoted to the access control (AC) channel, and thesubslots of even ranks F2, F4, S02, S04 terminate with 0.5 ms devoted tothe AC channel. The subslots of odd rank F1, F3 are thepresynchronization slots carrying the synchronization pattern FF, andthe subslots of odd rank S01, S03 are the synchronization supplementslots carrying the system information H, X, R and a bit at 0 todesignate the subslot S01 and at 1 to designate the subslot S03. Thisbit makes it possible to resolve the synchronization ambiguity resultingfrom the detecting of the pattern FF alone. Each of the synchronizationsupplement slots S01, S03 is preceded and followed by a subslot ofd₂/N=10 ms containing additional system information denoted Z. Thisinformation Z is therefore included by the base station in the subslotsF4, S02 and S04 since the composite slots F and S0 follow one another inthis example (p=1).

This procedure can be generalized to the case where N≧4. It relies onthe observation that a mobile terminal, even a simplified one, alwayspicks up at least two consecutive 10 ms periods in its monitoringwindow. A mobile terminal which has picked up the pattern FF in thefirst half of a monitoring window can then recover the additionalinformation Z in the second half of the next monitoring window (subslotS02 or S04), after it has received the system information in the subslotS01 or S03. Also, a mobile terminal which has picked up the pattern FFin the second half of a monitoring window can recover the additionalinformation Z in the first half of the next monitoring window (subslotF4 or S02), before it has received the system information in the subslotS01 or S03.

Thus, increased throughput is available in the guise of the controlchannel formed by the slots S0.

Returning to FIGS. 4 and 5, detailed hereinbelow is the structure of thesignaling channels associated with the traffic channels and sharing thesame carrier frequencies.

When the base station is currently listening to whatever the mobileterminal is transmitting on the carrier f_(TU), it has available atimeslot belonging to an associated logical signaling channel denoted DT(“downlink talker”) at the end of the first third of each frame on thecarrier f_(TD). The DT channel carries downlink signaling which may inparticular pertain to control of transmission power by the mobileterminal (power measurements made by the base station and allowing theterminal in a transmission phase to regulate its power so as to limitthe interference in the whole network), to indications of communicationsrelating to the terminal in a transmission phase, or else to commands tocease transmission (for example should the traffic channel be pre-emptedby a terminal of higher priority).

When the base station is currently transmitting to a mobile terminal onthe carrier f_(TD), it has available a timeslot belonging to anassociated logical signaling channel denoted DL (“downlink listener”) atthe end of the second third of each frame on the carrier f_(TD). The DLchannel carries downlink signaling which may in particular pertain toidentification (color codes) of the neighboring cells in which the groupcommunication is set up (allowing the terminals in a reception phase tochoose a new cell if the conditions of reception deteriorate), toindications of communications relating to the terminal in a transmissionphase, or else to transmission of the talker identity or of parametersserving for decrypting the signals transmitted on the traffic channel.

When the mobile terminal is currently listening to whatever the basestation is transmitting on the carrier f_(TD), it has available atimeslot belonging to an associated logical signaling channel denoted UL(“uplink listener”) at the end of the first third of each frame on thecarrier f_(TU). The UL channel carries uplink signaling which can inparticular pertain to random access from the terminal asking for thealternate operation entitlement, or else to responses to requests madeby the base station (on the logical channel DL) for ascertaining thepresence of the terminals.

When the mobile terminal is currently transmitting to a base station onthe carrier f_(TU), it has available a timeslot belonging to anassociated logical signaling channel denoted UT (“uplink talker”) at theend of the second part of each frame at the frequency f_(TU). The UTchannel carries uplink signaling which can in particular pertain tochange of cell requests if the terminal notes a deterioration in theradio conditions according to the measurements dispatched by the basestation on the logical DT channel or those made by the terminal, or elsea change of type of transmission request (for example from voice todata).

Of course, the various signaling elements exchanged on the channels DT,DL, UL and UT are not limited to those cited above by way of example.

The timeslots belonging to the associated channels DT, DL, UL, UT areelementary slots in mode 1 (FIG. 4), and composite slots in mode 2 (FIG.5).

As shown by FIG. 4, the elementary slots pertaining to the logicalchannels DT and UL are permuted within their composite slot in mode 1.The elementary slots appertaining to the logical channels DT and UL arerespectively even and odd, i.e. that of the uplink channel UL occursbefore that of the downlink channel DT, while the reverse holds in thecomposite slots pertaining to the downlink and uplink traffic channels.This disposition allows the simplified terminals, for which severalmilliseconds are necessary in order to change frequency, to access allof the 20 ms slots DT and UL.

Of course, if the odd elementary slots are reserved for the uplinkdirection and the even ones for the downlink direction in the logicaltraffic channels, then, in order to obtain the same result, provisionwill be made for the odd elementary slots to be reserved for thedownlink direction and the even ones for the uplink direction in thelogical signaling channels DT and UL.

As far as the slots DL and UT are concerned, these requiring no changeof frequency of the mobile terminals, the permuting of the elementaryslots within the composite slot which contains them is optional in mode1.

It should be noted that, in mode 1 with time duplex, there is nodistinction between the DT and DL channels, nor between the UT and ULchannels, since the terminal is simultaneously in the transmission phaseand in the reception phase on the duplex traffic channel.

In mode 2 (FIG. 5), the slots pertaining to the associated signalingchannels DT, UL, DL, UT are composite slots. The management of thechannels DL and UT does not pose any particular problem since (i) theyrequire no change of frequency of the mobile terminals, and (ii) onlythe transmitting mobile terminal is able to employ the channel UT.

To indicate the ninth timeslot of duration d₂ of a frame will be devotedto the downlink channel DT or to the uplink channel UL, the base stationuses a mechanism with control bits X₁, X₂, Y₁, Y₂ similar to thatdescribed earlier. The base station is then transmitting on the trafficchannel of the carrier f_(TD), so that it can insert the bits X₁, X₂,Y₁, Y₂ in the timeslots of the traffic channel so as to control accessto the ninth composite timeslot.

For example, the bits X₁, X₂ placed in the sixth timeslot of the frame(the case j=3) will indicate whether the mobiles are or are notauthorized to perform random access, i.e. whether the ninth slot belongsto an uplink (UL) or downlink (DT or DL) signaling channel, and the bitsY₁, Y₂ placed in the twelfth timeslot of the frame (the case j′=3) willapprise of the processing of any random access performed in the previousUL slot. The coding of the control bits X₁, X₂, Y₁, Y₂ can be the sameas before.

The ninth composite timeslot of the frame is therefore programmable bythe base station. Depending on needs, it will assign it either to theuplink UL channel or to a downlink channel. It should be noted that,when it is assigned to a downlink channel, the information transmittedin this slot may relate both to the terminals in the transmission phase(DT channel) and those in the reception phase (DL channel). This holdsfor mode 2 as just set forth, but also for mode 1: a simplified terminalcannot switch its frequency immediately between the elementary slotsdenoted UL and DT in FIG. 4, so that it is useful for the base stationto indicate to it whether the ninth slot will serve for the transmissionof downlink signaling (on the logical DL channel borrowing the slotdenoted DT) or for any transmission of uplink signaling (on the ULchannel).

It is very advantageous for the base station to have the possibility ofprogrammably increasing the overall throughput of the logical DL channelto the detriment of that of the logical UL channel. This allows goodadaptation to the needs encountered in the traffic communications underalternate operation, i.e. with successive alternations in which just oneof the mobile terminals is in the transmitting on the uplink logicaltraffic channel while one or more of the mobile terminals are receivingon the downlink logical traffic channel.

The base station can thus transmit access control bits X₁, X₂ suitablefor restricting the possibilities of random access by the mobileterminals at the start of an alternation (reduction in the overallthroughput of the UL channel), compared with the rest of thealternation. Indeed, there is little benefit in authorizing a newalternate operation pre-emption request only a short time after a changeof talker, while on the other hand the transmission of information onthe downlink DL channel is of particular benefit at precisely suchtimes. The assigning of additional slots to the channel DL at the startof a period of alternation thus enables information such as the identityof the talker or, in the case of an encryption system with externalsynchronization, the vector for initializing the decryption system, tobe transmitted as frequently as possible. The problems related to gapsdue to poor radio propagation conditions are thus addressed. Theeffectiveness and comfort for the user of the function of late entry toa group communication are also increased.

For the regularity of the structure of the timeslots, the control bitsX₁, X₂, Y₁, Y₂ can be inserted into each of the timeslots of the trafficframe on the carrier f_(TD) (as in the case of the control frame on thecarrier f_(CD)), even though just some of them suffice for coding theassignment of the ninth composite slot.

1. A method of transmitting radio signals between a base station andmobile terminals, wherein signals transmitted from the base station toat least one of the mobile terminals are transmitted on a firstfrequency and signals transmitted from at least one of the mobileterminals to the base station are transmitted on a second frequency, thetransmissions on the first and second frequencies being performedaccording to a frame structure comprising timeslots of like duration,wherein the first frequency carries a downlink logical traffic channelto at least one of the mobile terminals and the second frequency carriesan uplink logical traffic channel from one of the mobile terminals,wherein the first frequency further carries a first downlink logicalsignaling channel to one of the mobile terminals during the transmissionphase on the uplink logical traffic channel and a second downlinklogical signaling channel to at least one of the mobile terminals duringthe reception phase on the downlink logical traffic channel, wherein thesecond frequency further carries a first uplink logical signalingchannel from at least one of the mobile terminals during the receptionphase on the downlink logical traffic channel and a second uplinklogical signaling channel from one of the mobile terminals during thetransmission phase on the uplink logical traffic channel, and whereinthe first downlink and uplink logical signaling channels are eachcarried by a first timeslot of the frame structure, while the seconddownlink and uplink logical signaling channels are each carried by asecond timeslot of the frame structure, distinct from said firsttimeslot.
 2. The method as claimed in claim 1, wherein the framestructure comprises, in addition to said first and second timeslots,timeslots dedicated to the logical traffic channels, and at least onetimeslot forming a listening window in which the mobile terminals listento signals transmitted from other base stations.
 3. The method asclaimed in claim 2, wherein the signals transmitted from other basestations and which are searched for by the mobile terminals during thelistening window comprise synchronization patterns which are transmittedwith a periodicity of M times said duration of the timeslots, M being anumber prime to the number of timeslots of the frame structure.
 4. Themethod as claimed in claim 2, wherein each of the timeslots dedicated tothe logical traffic channels is a composite timeslot which comprises afirst elementary slot for the downlink logical traffic channel followedby a second elementary slot for the uplink logical traffic channel, andwherein the first timeslot carrying the first downlink and uplinklogical signaling channels is a composite timeslot which comprises afirst elementary slot for the first uplink logical signaling channelfollowed by a second elementary slot for the first downlink logicalsignaling channel.
 5. The method as claimed in claim 2, wherein each ofthe timeslots dedicated to the logical traffic channels is a compositetimeslot which comprises a first elementary slot for the uplink logicaltraffic channel followed by a second elementary slot for the downlinklogical traffic channel, and wherein the first timeslot carrying thefirst downlink and uplink logical signaling channels is a compositetimeslot which comprises a first elementary slot for the first downlinklogical signaling channel followed by a second elementary slot for thefirst uplink logical signaling channel.
 6. The method as claimed inclaim 4, wherein the second timeslot carrying the second downlink anduplink logical signaling channels respectively comprises two elementaryslots for the second downlink and uplink logical signaling channels. 7.The method as claimed in claim 2, wherein the frame structure comprisesa programmable timeslot scheduled for the transmission on at least thefirst uplink logical signaling channel, and wherein the base stationtransmits, during the timeslots dedicated to the logical trafficchannels, access control signals for controlling access to saidprogrammable timeslot by the mobile terminals in the reception phase. 8.The method as claimed in claim 7, wherein said access control signalscomprise signals for authorizing the mobile terminals in the receptionphase to transmit during a next programmable timeslot, and signals forapprising of the processing of requests made by the mobile terminals inthe reception phase during a previous programmable timeslot.
 9. Themethod as claimed in claim 7, wherein at least one programmabletimeslot, during which the mobile terminals in the reception phase arenot authorized to transmit, is assigned to the first downlink logicalsignaling channel.
 10. The method as claimed in claim 7, wherein atleast one programmable timeslot, during which the mobile terminals inthe reception phase are not authorized to transmit, is assigned to thesecond downlink logical signaling channel.
 11. The method as claimed inclaim 7, wherein the logical traffic channels are used in alternatemode, with successive alternations during which only one of the mobileterminals is in the transmission phase on the uplink logical trafficchannel while one or more of the mobile terminals are in the receptionphase on the downlink logical traffic channel, and wherein the basestation transmits access control signals in such a way as to restrictthe transmissions of the mobile terminals during the programmabletimeslots in an initial period of an alternation with respect to therest of the alternation.
 12. A radiocommunication base station,comprising means for transmitting radio signals on a first frequency toat least one mobile terminal, and means for receiving radio signals on asecond frequency from at least one mobile terminal, the transmissions onthe first and second frequencies being performed according to a framestructure comprising timeslots of like duration, wherein the firstfrequency carries a downlink logical traffic channel from the basestation, wherein the second frequency carries an uplink logical trafficchannel to the base station, wherein the first frequency further carriesa first downlink logical signaling channel to a mobile terminal duringthe transmission phase on the uplink logical traffic channel and asecond downlink logical signaling channel to at least one mobileterminal during the reception phase on the downlink logical trafficchannel, wherein the second frequency further carries a first uplinklogical signaling channel from at least one mobile terminal during thereception phase on the downlink logical traffic channel and a seconduplink logical signaling channel from a mobile terminal during thetransmission phase on the uplink logical traffic channel, and whereinthe first downlink and uplink logical signaling channels are eachcarried by a first timeslot of the frame structure, while the seconddownlink and uplink logical signaling channels are each carried by asecond timeslot of the frame structure, distinct from said firsttimeslot.
 13. A radiocommunication mobile terminal, comprising means forreceiving radio signals on a first frequency from a base station, andmeans for transmitting radio signals on a second frequency to the basestation, the transmissions on the first and second frequencies beingperformed according to a frame structure comprising timeslots of likeduration, wherein the first frequency carries a downlink logical trafficchannel to the mobile terminal, wherein the second frequency carries anuplink logical traffic channel from the mobile terminal, wherein thefirst frequency further carries a first downlink logical signalingchannel to the mobile terminal during the transmission phase on theuplink logical traffic channel and a second downlink logical signalingchannel to the mobile terminal during the reception phase on thedownlink logical traffic channel, wherein the second frequency furthercarries a first uplink logical signaling channel from the mobileterminal during the reception phase on the downlink logical trafficchannel and a second uplink logical signaling channel from the mobileterminal during the transmission phase on the uplink logical trafficchannel, and wherein the first downlink and uplink logical signalingchannels are each carried by a first timeslot of the frame structure,while the second downlink and uplink logical signaling channels are eachcarried by a second timeslot of the frame structure, distinct from saidfirst timeslot.
 14. The method as claimed in claim 5, wherein the secondtimeslot carrying the second downlink and uplink logical signalingchannels respectively comprises two elementary slots for the seconddownlink and uplink logical signaling channels.